A lithium ion secondary battery using lithium titanate such as Li4Ti5O12 as a negative electrode active material has been recently developed. In such a lithium ion secondary battery, it is known that carbon dioxide is adsorbed as an impurity at the negative electrode. Carbon dioxide may react with the negative electrode active material and thereby, produce a large amount of gas. Such a reaction is remarkable particularly when the battery was stored at a temperature equal to or higher than room temperature. If a large amount of gas is generated, the battery internal pressure increases. Further, in such a case, plastic deformation of the battery may occur.
In addition, it is known that an SEI (Solid Electrolyte Interface) coating (hereinafter, referred to as a coating) is formed on the surface of a negative electrode active material of a lithium ion secondary battery. This coating is produced mainly due to the reductive decomposition of a non-aqueous electrolyte. Reaction between an active material and the non-aqueous electrolyte can be suppressed by this coating. However, when the coating is too thick, diffusion resistance of the lithium ion becomes larger. As a result, the large current performance is reduced. On the other hand, when the coating is too thin, the reaction of the non-aqueous electrolyte with the active material is less likely to be suppressed. As a result, self-discharge during storage of the battery is increased.